Fresh drinks (for example fresh milk or pressed fruit juices) are nowadays very often sold in plastics containers. Such containers are frequently in the form of a bottle with an appropriate screw cap. Suitable materials for the production of such plastics containers are in particular polypropylene (PP) or polyethylene (PE), it also being possible to use many other materials such as, for example, PET (polyethylene terephthalate). The same materials are conventionally also used to produce screw caps. In the case of certain other products, cans are provided which are closed by attachable lids which fit onto the opening. Such lids are often referred to as snap-on lids because they form a snap fit with the flange of the container opening.
It is particularly important in the case of such fresh products that they are able to remain in the perfect state from the time of packaging to the time of consumption. It has been shown in particular that contact of the products with oxygen from the ambient air very quickly leads to various reactions and consequently to a negative change in the fresh foodstuffs. Although many of these changes do not actually impair the edibility of the products, they nevertheless lead to a marked change in the taste, smell, colour or consistency, which adversely affects the sale of such products.
Because conventional screw caps for plastics bottles or snap-on lids for cans are typically not air-tight, various measures have already been proposed to ensure that such fresh foodstuffs are sealed in an air-tight manner. One of the most frequently used measures is an air-tight seal of the container opening, which must first be removed prior to consumption. There is typically used for that purpose an aluminium-containing foil, the ends of which are attached in an air-tight manner to the opening of the container by a special soldering process. The container is then closed by the screw cap. However, such an additional foil involves numerous disadvantages, in particular because the packaging process in the case of such containers is made more difficult, and accordingly also more expensive, by the addition of a supplementary step. However, such a closure is also associated with disadvantages for the consumer, because the foodstuffs cannot be consumed before the protective film is removed. In addition, the protection from air is completely eliminated once the protective film has been removed, so that foodstuffs which have not been used up fully can no longer be stored for very long.
Ethylene vinyl alcohol copolymer (EVOH) is a material, or a group of materials, which is extremely suitable for air-tight sealing because it has outstanding gas-barrier properties. In addition, EVOH copolymers are resistant to oils and organic solvents and can additionally be applied very easily to other plastics, with which they form strong composites. These advantageous properties have led to EVOH being used in the packaging industry, especially in the field of medical, pharmaceutical and cosmetic products. In addition, the practice has recently begun of forming either an EVOH coating or an EVOH intermediate layer in the case of screw caps produced from conventional plastics. The reason for this is that such an additional EVOH layer has the effect that containers can be sealed in an air-tight manner using only the cap, as a result of which additional protective films are completely unnecessary. Packaging processes can thus also be simplified, so that the products can be sold substantially more cheaply. Such caps also provide protection against the ambient air even after the containers have been opened, which cannot be ensured in the case of the conventional methods.
However, for reliable air-tight sealing, the EVOH layer must not have any gaps, so that EVOH not only has to be present but also must be free of gaps and faults and have a sufficient layer thickness. This is because gaps in the EVOH layer can lead to the ambient air being able to enter the container despite the seal, as a result of which the above-described problems relating to the change in the fresh products occur. Unfortunately, however, there are at present no satisfactory inspection methods by means of which screw caps with an EVOH layer can be inspected on an industrial scale. The reason for this is that EVOH layers cannot be distinguished visually from the caps, so that the inspection of such coated screw caps with the aid of conventional inspection methods is not possible. For this reason, such caps are unfortunately not widely used.
From WO 2007/021551 A1 there is known a method for determining the authenticity of packaging for pharmaceutical material. These original packagings consist of PP or HDPE and approximately 5% added EVA. In this method, a spectrum of the packaging material is recorded by means of an FTIR spectrometer, and conclusions are drawn from the presence or absence of specific peaks as to the presence or absence of EVA, and accordingly as to an original packaging.
From US 2002/0033943 A1 there is known a method for determining the quality of objects made of coloured plastic, in which a camera operating in visible light and near-infrared is used to detect bubbles, particles or crystalline regions in the wall material or to determine filling levels in dark containers. To that end, the object made of PP, PE or EVOH is irradiated with infrared light of from 700 nm to 2000 nm, and reflected, scattered or unabsorbed light is detected.